The present invention relates generally to a mechanism for mapping fracture systems in subterranean gas- and oil-bearing earth formations, and more particularly to such a mechanism which determines directional characteristics of a hydraulically induced fracture in earth formations through the use of and by implacing, transmitting and receiving antennas in the fracture to provide signals indicative of the length and orientation of the fracture.
Inducing fractures in subterranean earth formations has proven to be an important technique for increasing the recovering of energy values contained in the earth formation. In order to strategically place wells in earth formations to assure maximum gas or oil recovery from fractured formations during enhanced oil-recovery practices, the length and directional characteristics of the fracture need to be accurately determined. Several techniques have been utilized to map and characterize fractures during or subsequent to the formation of the fractures. The monitoring of acoustic and seismic emission during fracture propagation from surface sites or sensors in adjacent wellbores are typical of the systems utilized to map fracture characteristics. However, several conditions such as background and surface noise, lack of existing wells in area, low energy level of seismic signals emitted, prohibitive costs of providing new closely spaced wellbores for the purpose of fracture mapping, and data too complicated to analyze or which would require the expenditure of too much time, money and sophisticated equipment for the analyses significantly reduce the accuracy of such mapping techniques. Impression packers have been frequently used in wellbores to indicate the direction of fracture propagation from the wellbore. However, since the fracture orientation may abruptly change a short distance which may be as little as a few inches from the wellbore the information provided by using impression packers may not be sufficiently accurate for many purposes. All other methods of mapping subsurface fractures from the surface are indirect since they actually measure surface upheavel (vertical displacement and slope change). Such methods yield minimal solutions and due to the inherent filtering of any electrical or seismic signal result in very low accuracy and resolution. Surface and subsurface borehole tiltmeters fall into this category. The same problems also exist with the method of measuring the fracture induced electrical potential change by the use of surface electrical potential measuring arrays. All known methods use indirect methods of mapping fractures since such fractures have heretofore been considered inaccessible.